1. Field of the Invention
The present invention relates to a sheet discharge roller employed in a sheet exit port of an image forming apparatus as well as an image forming apparatus provided with the sheet discharge roller.
2. Description of the Related Art
For improving sheet discharging performance for discharging paper sheets, processing of increasing rigidity of the paper sheet is performed by slightly curving the paper sheet in a direction (i.e., a widthwise direction of the paper sheet) perpendicular to a sheet discharging direction when the paper sheet is discharged from an exit port of an image forming apparatus. As a method for increasing rigidity of the paper sheet, there has been a method in which a rigidity increasing portion independent of a transportation roller (i.e., a roller for transportation) is arranged coaxially to the transportation roller.
The rigidity increasing portion has a larger outer diameter than the transportation roller, and is configured to deform the paper sheet with respect the widthwise direction of the paper sheet into a wavy form when viewed in a sheet discharging direction so that it increases the rigidity of the paper sheet in the sheet discharging direction and improves the paper sheet discharging performance.
Japanese Laid-Open Patent Publication Nos. 09-301590 and 2003-040507 are prior art references that have disclosed the above kind of sheet discharge rollers employing the rigidity increasing portions. In these references, the rigidity increasing portion employs a structure allowing removal from a rotation shaft.
In recent years, use of the rotation shaft molded of resin has been proposed and employed from a viewpoint of cost reduction of the image forming apparatus. In this case, a roller portion of a transportation roller and a rigidity increasing portion are formed integrally with the rotation shaft, and these are molded of resin.
A method of integrally forming the roller portion and the rigidity increasing portion on the rotation shaft is effective in cost reduction. However, it causes the following problems. For implementing the function as the roller for transportation, the roller portion molded of resin is provided at its surface with an annular rubber member attached thereto. A friction between the rubber member and the paper sheet provides a sheet transportation force. Although the rubber member is employed as an annular elastic member, this is not restrictive and, for example, an elastomer member or the like having similar characteristics may be used.
FIG. 21 shows a sheet discharge roller 9 in which roller portions and rigidity increasing portions are formed integrally with a rotation shaft. A rotation shaft 91 is provided with roller portions 93a axially spaced from each other by a predetermined distance. An annular rubber member 93b is attached to a surface of roller portion 93a. Roller portion 93a and rubber member 93b form a transportation roller 93.
Rotation shaft 91 is also provided with rigidity increasing portions 92 spaced from each other by a predetermined distance and each having a substantially cylindrical form. When viewed in the axial direction, an outer diameter of rigidity increasing portion 92 is larger than an outer diameter of transportation roller 93.
For combining rubber member 93b to roller portion 93a of sheet discharge roller 9 having the above structure, an elastic force of rubber member 93b is utilized, and rubber member 93b is radially expanded to have a diameter φD) equal to or larger than an outer diameter (φd) of rigidity increasing portion 92. However, rubber member 93b can be radially expanded only to a limited extent, and restricts the outer diameter of rigidity increasing portion 92 that is larger than the outer diameter of transportation roller 93. Conversely, the larger outer diameter of rigidity increasing portion 92 can increase the rigidity of the paper sheet.
As described above, it is preferable to reduce the outer diameter of rigidity increasing portion 92 from the viewpoint of the attaching rubber member 93b to roller portion 93a in the case where roller portion 93a and rigidity increasing portion 92 are integrally formed on rotation shaft 91. Conversely, it is preferable to increase the outer diameter of rigidity increasing portion 92 for increasing the rigidity of the paper sheets. Sheet discharge roller 9 must satisfy these conflicting requirements.